Surface mounting of electronic components is well developed in automated package assembly systems. Interface adhesives are used in several approaches to provide lid attach, sink attach and mainly thermal transfer from flip chip devices, as well as against mechanical shock and vibration encountered in shipping and use. As semiconductor devices operate at higher speeds and at lower line widths, the thermal transfer properties of the adhesive are critical to device operation. The thermal interface adhesive must create an efficient thermal pathway between the die or lid and the heat sink as the adhesive itself due to interface resistance (θint) and bulk resistance (θadh) is typically the most thermally resistant material in the die-adhesive-lid-adhesive-sink or die-adhesive-sink configuration. The thermal interface adhesive must also maintain efficient thermal transfer properties through reliability testing which simulates actual use conditions over the life of the device. Reliability testing includes exposure for a specified time to a 130° C., 85% relative humidity environment, exposure to an 85° C., 85% relative humidity environment and high temperature storage at 125° C. to 160° C. The adhesive must not delaminate from the substrates or the bulk thermal resistance of the adhesive degrade after exposure to the reliability testing, thereby causing failure of the package. The interface may be applied after the reflow of the metallic or polymeric interconnect and after curing the underfill. A measured amount of interface adhesive will be dispensed usually on the die surface and on the periphery of the carrier substrate in a lidded flip chip assembly (θjc). The adhesive may also be dispensed on the top of the die surface and the heat sink placed in a die-to-sink application (θja). Additionally, the adhesive can be dispensed on the lid surface and the heat sink placed in a sink-to-lid application (θca). After the adhesive is dispensed, the adherends are placed with a predetermined pressure and time. The assembly is then heated to cure the adhesive.
Curable adhesives have been made using polyamide resins and epoxy resins as in U.S. Pat. No. 2,705,223 (Renfrew et al.). But the Renfrew compositions possess inferior properties when applied as adhesives. For example, the Renfrew compositions do not possess good adhesive strength upon cure and provide limited working time after the mixing of the components. In addition, such compositions exhibit poor flexibility, and poor adhesive resistance to heat, water and organic solvents when applied to substrates at ambient temperature.
U.S. Pat. No. 3,488,665 (MacGrandle et al.) teaches a process wherein polyamides are blended with epoxies to provide a product which cures after application to the substrate; however, the product is used to provide a hard, stiffer coating for flexible sheet material.
U.S. Pat. No. 4,082,708 (Mehta) teaches an adhesive system comprising an epoxy resin and a polyamide wherein the polyamide is derived substantially from primary and tertiary amines; specifically, the Mehta polyamides are derived from 1,4-bis-primary amino lower alkyl piperazine having terminal primary amine groups. Although it is suggested that secondary amines can be utilized in making the polyamides as chain extenders and flexibilizers, it is taught that the secondary amines are less desirable reactants and should be buried within the polyamide structure.
U.S. Pat. No. 3,257,342 to J. N. S. Kwong teaches a thermoset epoxy resin composition comprising a polyglycidyl ether, an amino-terminated polyamide of (a) polymeric fatty acids and aliphatic dicarboxylic acid and (b) a polyoxyalkylenediamine. Particularly preferred are dimer fatty acids or mixed dimer and trimer acids.
U.S. Pat. No. 4,070,225 to V. H. Batdorf describes a latent or slow-curing adhesive system formulated from an epoxy resin and a primary amine-terminated polyamide. The polyamide was prepared from a polymeric tall oil fatty acid, a polyoxypropyleneamine, 1,4-bis-aminopropyl piperazine, and ethylenediamine.
U.S. Pat. No. 4,082,708 to R. Mehta describes polyamide curatives of epoxy resins prepared from 1,4-bis(3-aminopropyl)piperazine, dimerized tall oil oil fatty acids, polyoxypropylenediamine, and ethylenediamine or piperazine.
The polyamide was used with EPON® 828 as a metal-to-metal adhesive.
U.S. Pat. No. 5,189,080 discloses an encapsulating resin for electronics components consisting of cycloaliphatic epoxy resin, a hardener, an accelerator, a filler, and optionally, a pigment. The hardener can be methylnadic anhydride and the filler can be amorphous silica.
U.S. Pat. No. 5,248,710 discloses flip chip encapsulating compositions comprising an difunctional epoxy resin, a silicone-modified epoxy resin, an imidazole curing agent soluble in epoxy resin, and fused silica filler.
U.S. Pat. No. 5,416,138 discloses epoxy resin compositions for sealing semiconductor devices, essentially, (A) an epoxy resin containing 50-100 weight percent of a diglycidyl ether of a substituted bisphenol (B) a phenolic resin curing agent containing 30-100 weight percent of a flexible phenolic resin curing agent, (C) an inorganic filler and (D) a curing accelerator.
U.S. Pat. No. 5,439,977 discloses an acid anyhdride-containing one package epoxy resin composition consisting indispensably of (1) an epoxy resin having two or more epoxy groups per molecule, (2) an acid anhydride, (3) at least one of (a) a liquid latent curing accelerator, (b) a latent curing accelerator soluble in epoxy resins (c) a latent curing accelerator soluble in acid anhydride and (4) a dispersible latent curing accelerator.
There are a plethora of latent amine curing agents derived from different functional compounds taught for epoxy resin systems. Among these are the blocked or hindered imidazoles, polyamidoamines, polyamide condensates of dimer acids with diethylenetriamine, triethylenetetramine, and aromatic polyamines.
U.S. Pat. No. 6,046,282 discloses a reduced viscosity reactive diluent which is a blend of a high viscosity polyamidoamine epoxy curative and 5 to 95 wt. % of a lower reactive diluent comprising the amide and/or imidazoline reaction product of a polyalkylene polyamine and a C2-C7 aliphatic monocarboxylic acid or a C7-C12 aromatic monocarboxylic acid.
U.S. Pat. No. 6,214,460 discloses a screen-printable adhesive composition capable of being applied to a substrate at room temperature comprising at least one alkyl acrylate; at least one reinforcing comonomer, a polyepoxide resin, and a amine adduct with epoxy resin, alkylene epoxides, acrylonitrile, or condensation products with fatty acids or Mannich bases.
U.S. Pat. No. 5,575,596 discloses a rheologically stable, thermal curing flexible electrically-conductive epoxy-based adhesive composition comprising: (a) a polymer mixture comprising at least one polyepoxide resin having a cured hardness not exceeding a Durometer Shore A of 45 and cured with a stoichiometric amount of diethylene triamine, and substantially stoichiometric amount of at least one latent epoxy resin curing agent; and (b) an electrically-conductive filler comprising a metal.
In light of the foregoing, the design of integrated circuit packages heat dissipation is limited by thermal interface materials. Long-term quality defects can arise from many types of failures in the adhesive. Corrosion of nickel-coated copper tungsten lids and aluminum sputtered nickel-coated copper tungsten lids is also a recurring problem. Improvements in thermal interface materials of the 100% solids thermosetting epoxy adhesive type, especially in bond-lines ranging from 75-125 microns would be industrially important to enable higher power loads demanded.
A suitable adhesive must have certain fluid handling characteristics, and exhibit specific adhesion, controlled shrinkage, and low corrosivity in order to provide long term defect-free service over the thermal operating range of the electronic circuit package. Whereas desired properties for thermal interface materials are known such as sharp, well-defined, stable and reproducible Tg, an initial high and stable electrical conductivity, ability to withstand high temperatrue and voltage during repeated “switching” cycles without loss of any of these properties, adhesives fulfilling all of the requirements are not easily found. Wheras it is known to introduce latent thermal curatives to ensure that voids are not left after cure by allowing heating of the resins under vacuum for prolonged periods of time without causing premature gelation, the effects of the crosslinked polymer network on achieving improvements in thermal conductivities is not well understood.
A drawback to highly filled thermosetting epoxy resin compositions currently used in microelectronics applications, such as for underfills, is their extended cure schedule and useful working life at dispensing temperatures and ability to remain at a stable viscosity until curing is initiated. In modern continuous dispensing processes via mechanical pumping devices where a pump has been attributed to the adhesive.
It would be industrially important to provide a 1-part, high solids, thermosetting adhesives adapted to exhibit a controlled shrinkage after curing, and improved post-cure thermal conductivity of 10 W/mK, and above.
A variety of thermally conductive fillers have been employed conventionally with epoxy resins containing solvents and approach or exceed this level of bulk thermal conductivity, however a relatively higher solvent level contributes to worsening shrinkage, void formation, and delamination. Conventional syringe dispensing techniques include time-pressure, positive displacement, and auger-type valve technologies.
It has been found that in a system containing less than 4% volatile components, at a high filler level (70 wt. % and above) in a liquid epoxy adhesive, dispensing equipment malfunctions and is thought to result from a loss of fluidity of a portion of the highly filed epoxy adhesive within the dispensing device.
Provided that a stable adhesive can remain free flowing during continuous dispensing, a balance of properties in the cured solid-phase thermal interface adhesive are needed and are affected by the polymer composition. Besides maintaining filler level above 70 wt. % in a free flowing fluid, the organic components also contribute significantly to the resulting cured thermal conductivity, shrinkage, coefficient of thermal expansion (CTE) and therefore essential for long term, defect-free service in the assembled devices after thousands of temperature cycled from as low as −55° to as high as 125° C.
Certain of the technical problems associated with providing a 1-part curable epoxy adhesive capable of being dispensed to obtain a thin bond-line between the die and lid, or between the lid and heat sink of from about 20 μ to 150 μ are overcome by the present invention. Heretofore, bulk thermal conductivity above 10 w/m° K. can be achieved in highly filled thermoset epoxy formulations by the addition of non-reactive solvent, plasticizer or other liquid viscosity reducing diluent, but inclusion of such unreactive diluents result in unacceptable percent shrinkage. It would be industrially important to provide a low viscosity, 100% solids material essentially devoid of solvents, hence low in shrinkage but exhibiting a bulk thermal conductivity of greater than 4 W/m° K., and preferably greater than 10 W/m° K. while exhibiting a volume resistivity of less than 200 mΩ-cm, and preferably up to 100 mΩ-cm.
The critical fluid properties for high speed dispensing of thermal interface embodiments are: viscosity less than about 10,000 poise measured using a Haake® RS1 cone and plate controlled stress rheometer at 25° C. at 2.0 sec−1 using a 1°, 35 mm cone. The preferred viscosity in accordance with the invention was observed in a range of from 1200 poise to 6000 poise at 2.0 sec−1. The thixotropic index as the ratio of viscosity at 0.2 sec−1 to viscosity at 2.0 sec−1 is in a range of from 3 to about 7. In a 24-hour period at 25° C. the invention exhibits a viscosity stability of less than 30% viscosity variation over 24 hours. The invention provides sufficient flow and wetting of the dispensed adhesive material to the parts to be bonded when dispensed from a syringe or printed utilizing a screen printer, as practiced on conventional automated assembly lines.